Robert Mitchum's character was wounded and needed to use a crutch, but Mitchum would switch which arm he used with the crutch throughout shooting. The continuity was so poor that John Wayne (who actually worked continuity in silents while a star college football player, a method used by Hollywood fans to slip players some spending money) had his character mention it in one of the last scenes. Director Howard Hawks enjoyed it so much he left it in the movie. Mitchum's version of this story is that he objected but Hawks had him switch sides with the crutch based on what looked best in that scene. When Hawks saw how bad it looked in the dailies, Mitchum suggested the additional dialogue between his character and Wayne's to cover the gaffe.
The opening credits feature a montage of original paintings that depict various scenes of cowboy life in the Old West. The artist was Olaf Wieghorst, who appears in the film as the Gunsmith, Swede Larsen.
The ingredients that Mississippi recites for Johnny Diamond's recipe to sober up J. P. Hara are: cayenne pepper, hot mustard powder, ipecac, asafoetida, and croton oil. Ipecac is a strong emetic, asafoetida is a spice known for its strong sulfurous odor, and croton oil is a potent purgative. Anyone who administered this combination in real life would likely be shot a day or two later when the patient could finally leave the outhouse, assuming the unfortunate victim had not died of dehydration from the violent fluid diarrhea croton oil causes.
Arch-conservative John Wayne did not get along with actor Edward Asner, whose politics were quite liberal, during filming, and constantly referred to Asner as "that New York actor". Asner later said he did not know if the phrase was intended to be an anti-semitic insult.
The poem "El Dorado" has four verses. James Caan's character recites three, omitting the second, which laments the aging knight's failure to locate El Dorado. He recites the first verse and part of the fourth riding with John Wayne after they meet for the first time, the third when Wayne is about to ride out for the final gunfight, and the complete fourth when he himself takes up the second wagon's reins.
Robert Mitchum revealed in an interview that when Howard Hawks asked him to be in the film, Mitchum asked what was the story of the film. Hawks reportedly replied that the story didn't matter because the film had some "great characters".
John Wayne was disappointed that the movie was released at the same time as his next movie, The War Wagon (1967). However, despite this film receiving generally poor reviews and being seen as old-fashioned and out of tune with the times, both movies proved to be hugely successful at the box office.